Electrospray ionization (ESI) is a method of generating ions in the gas phase at relatively high pressure. ESI was first proposed as a source of ions for mass analysis by Dole et al. (J. Chem. Phys. 1968, 49, pp. 2240-2249). The various teachings of Fenn et al. (J. Phys. Chem. 1984, 88, pp. 4451-4459; J. Phys. Chem. 1984, 88, pp. 4671-4675; Anal. Chem. 1985, 57, pp. 675-679) helped to demonstrate the potential of ESI for mass spectrometry. Since then, ESI has become one of the most commonly used types of ionization techniques due to its versatility, ease of use, and effectiveness for large biomolecules.
ESI involves applying a high electric potential to a liquid sample flowing through a capillary (herein referred to as a sprayer). Droplets from the liquid sample become charged and an electrophoretic type of charge separation occurs. In positive ion mode ESI, positive ions migrate downstream towards the meniscus of the liquid at the tip of the capillary. Negative ions are attracted towards the capillary and this results in charge enrichment. Subsequent fissions (Schmeizeisen-Redeker et al., Int. J. Mass Spectrom. Ion Processes, 1989, 90, pp. 139-150) or evaporation (Iribarne et al., J. Chem. Phys., 1976, 64, pp. 2287-2294) of the charged droplet result in the formation of single solvated gas phase ions (Kebarle et al., Anal. Chem., 1993, 65, pp. 972A-986A). For mass spectrometry, these ions are then usually transmitted to the aperture of a downstream analysis device such as a quadrupole mass spectrometer, a time-of-flight mass spectrometer, an ion trap mass spectrometer, an ion cyclotron resonance mass analyzer, an electric sector, a magnetic sector or the like.
Ion spray ionization is a form of ESI in which a nebulizer gas flow is used to promote an increase in droplet fission (Bruins et al., Anal. Chem., 1987, 59, pp. 2642-2646). The nebulizer gas aids in the break-up of droplets formed at the capillary tip. Ions formed in this manner can be directed into the first chamber of various mass spectrometers which include, but are not limited to, quadrupoles, time-of-flight, ion traps, ion cyclotron resonance, and sector mass spectrometers. In addition, the nebulizer gas flow may be heated (Turbo IonSpray™) to aid in desolvation of the charged droplets.
In either electrospray or ion spray ionization, an ion spray is a spray of ionized or charged droplets that are generated from an ion source. The ion source may be a sprayer comprising a capillary which is provided with a sample from which the ions are generated. The capillary is further adapted to have an electric potential applied thereto. In addition, the sample flow rate through the capillary may vary. In some cases, the sample flow rate may be reduced to the order of hundreds of nanoliters per minute in which case the sprayer is referred to as a reduced flow-rate sprayer. The sample flow-rate may further be reduced to the order of nanoliters per minute in which case the sprayer is referred to as a nanosprayer. The sprayer may further have a heated element to provide heat to a nebulizer gas which may be provided to the sprayer in the case of a Turbo IonSpray™ ion source.
In mass spectrometry, considerable time is wasted in performing multiple analyses while samples are manipulated in upstream processing. For example, in high-performance liquid chromatography mass spectrometry (HPLC-MS), the samples must first be separated. Accordingly, analytes of interest may only elute from a sample within a narrow time window that is 15-18 minutes after the start of an HPLC-MS analysis (which may last for 20 minutes). Therefore, a conventional HPLC-MS system equipped with a single sprayer collects meaningless data for the first 15 minutes and the last 2 minutes of each analysis. This inefficient use of time is compounded for laboratories which analyze thousands of samples per week.
To address this issue, a mass spectrometer with an ion source employing multiple sprayers (i.e. a multisprayer ion source) may be used for multiple analyses by staggering the start time of each analysis. For example, four HPLC-MS analyses may be staggered in a mass spectrometer with an ion source having four sprayers by commencing the first analysis at time t0, the second analysis at time t0+5 minutes, the third analysis at time t0+10 minutes and the fourth analysis at time t0+15 minutes. The analytes of interest will then be sampled from the first sprayer between 15 to 18 minutes after time t0, from the second sprayer between 20 to 23 minutes after time t0, from the third sprayer between 25 to 28 minutes after time t0 and from the fourth sprayer between 30 to 33 minutes after time t0. In this fashion, 13 analyses may be conducted within 80 minutes. In contrast, a mass spectrometer with an ion source having one sprayer will only permit 4 analyses to be conducted within the same 80 minute time frame.
Multisprayer ion sources require control of each sprayer for high-throughput operation and to facilitate any desired test protocol in which sprayers are simultaneously operated, sequentially operated or any combination thereof. Accordingly, various techniques for controlling multiple sprayers have been disclosed in the prior art. For instance, Andrien et al. (WO 99/13492) disclose an apparatus having several sprayer probes (i.e. sources) for introducing multiple samples and calibration solutions into an atmospheric pressure ion source for mass spectrometry. Andrien et al. state that the mixture of samples and/or solvents may be sprayed simultaneously or individually in a variety of combinations. To turn off the ion spray generated by a sprayer, Andrien et al. turn off the sample delivery system that provides the sample solution flow to that sprayer. Andrien et al. further disclose that applying an appropriate potential to the tip of the sprayer may be used to disable the sprayer. Andrien et al. also state that if a reservoir is used as a sample solution source, the liquid flow to the sprayer may be controlled by turning the nebulizer gas flow on or off.
However, using the sample delivery system or the nebulizer gas flow to disable or re-enable a sprayer may require several seconds. For instance, when the sample delivery system is used to disable a sprayer, the ion spray generated by the sprayer continues until the sample solution has completely drained from the transfer capillary leading to the sprayer due to residual pressure within the capillary. This problem is compounded for ion sources operating at very low sample solution flow rates. There are also situations in which shutting down the nebulizer gas flow only affects ion spray stability and does not disable a sprayer.
Furthermore, when the voltage applied to a sprayer is used to disable and re-enable a sprayer, there are undesirable effects such as time delays for sprayer stabilization due to changes in applied voltage. In addition, a droplet may form at the sprayer tip, when the voltage applied to the sprayer is turned off, which will impede the sprayer from immediately generating an ion spray with the re-application of a potential to the sprayer.
Another prior art method to control sprayers in a multisprayer ion source involves having each sprayer generate an ion spray that enters a downstream mass spectrometer via multiple inlet apertures and then utilizing an electric field within the mass spectrometer to deflect the ion sprays towards or away from further stages of the mass spectrometer. This is done by placing an electrode downstream from the entrance aperture of the mass spectrometer and applying an appropriate potential to either transmit or deflect ions as disclosed by Kato (JP2000/357488) and Covey (WO 01/44795). For this method, Covey teaches that sprayer stabilization is not an issue since the sprayers are always on.
These types of mass spectrometers are effective for the elimination of sample carry-over from one sprayer to the next. However, the vacuum pumping requirements and associated costs for these mass spectrometers can become very large when multiple inlet apertures are installed onto the mass spectrometer. Hence, the number of sprayers and inlet apertures is limited by the vacuum chamber pumping requirements of the mass spectrometer. It is also not apparent how the focusing/deflecting electrode within the first vacuum stage affects the overall sensitivity of the mass spectrometer.
Another approach in the prior art to control sprayers in a multisprayer ion source involves moving a selected sprayer in front of the inlet aperture of a mass spectrometer. For instance, Hindsgaul et al. (WO 99/50667) disclose mounting a plurality of sprayers on a wheel and rotating the wheel in front of the inlet aperture of a mass spectrometer. Alternatively, Hannis et al. (J Am Soc Mass Spectrom 2000, 11, pp. 876-883) disclose a dual ion source having two sprayers and a solenoid that is actuated to line up one of the two sprayers in front of the heated capillary inlet of a downstream mass spectrometer.
Another mechanical method involves keeping all sprayers generating ions continuously and employing a mechanical device to block the ion spray from each sprayer except for one sprayer which is aligned with an aperture contained in the blocking device as disclosed by Hindsgaul et al. (WO 99/50667) and Covey et al. (WO 01/44795). Other devices incorporating this concept are disclosed in Wang et al. (Comb. Chem. High Throughput Screening, 1999, 2, pp. 327-334), De Biasi et al. (Rapid Commun. Mass Spectrom., 1999, 13, pp. 1 165-1168), and Wolff et al. (Anal. Chem., 2001, 73, pp. 2605-2612).
Although, mechanical devices are more effective for selecting the ion spray generated by a given sprayer than varying sprayer potential or sample solution flow rate, mechanical devices tend to decrease the overall sensitivity of the ion source (Yang et al., Anal. Chem., 2001, 73, pp. 1740-1747). There is also the possibility for sample carry-over from one sprayer to the next. Mechanical systems are also prone to reliability concerns. Furthermore, Covey et al. (WO 01/44795) stated that mechanical methods suffer from the time delay incurred from the mechanical positioning of the blocking device and that excessive liquid impacting a rotating mechanical device may result in excessive background interferences.